This invention relates to a process for depositing a first coating of substantially pure tin and a second or subsequent cap layer of one or more alloys of at least two immersion platable metals selected from tin, silver, bismuth, copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium and cobalt onto a metal surface such as copper or copper alloys used, e.g., for electrodes or various electronic circuit elements. More particularly, this invention relates to immersion plating of a first coating of substantially pure tin and at least one cap layer of an alloy comprising at least two of the above immersion platable metals on copper, copper alloys, and other metals on printed circuit boards by chemical displacement using an electroless immersion, spray, flood or cascade plating application process. Still more particularly, this invention relates to the use of such plating solutions in the manufacture of printed circuit boards.
Coatings of tin, lead, bismuth, and alloys thereof have been applied to surfaces of copper and copper-based alloys by displacement plating such as by immersion plating techniques. Chemical displacement plating has been used in the manufacture of printed circuit boards (PCB""s) and particularly multilayer printed circuit boards. Printed circuit boards generally comprise a non-conducting or dielectric layer such as a fiberglass/epoxy sheet which is clad with a metal conductive layer such as copper or a copper alloy on one or both surfaces. The metal layer on the PCB, before processing, typically is a continuous layer of copper which may be interrupted by a pattern of plated through-holes linking both surfaces of the board. During processing, selected portions of the copper layer are removed to form a raised copper circuit image pattern, i.e., circuitry. Multilayer PCB""s are typically constructed by inter-leaving imaged conductive layers such as one containing copper with dielectric adhesive layers such as a partially cured B-stage resin, i.e., a prepreg, into a multilayer sandwich which is then bonded together by applying heat and pressure. Production of these types of PCB""s is described in xe2x80x9cPrinted Circuits Handbook,xe2x80x9d Fourth Edition, Edited by C. F. Coombs, Jr., McGraw-Hill, 1996, which is incorporated herein by reference. Since the conductive layer with a smooth copper surface does not bond well to the prepreg, copper surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich.
In addition to problems associated with adhesion of copper to laminating materials, PCBs also must be treated in order to preserve the solderability of exposed conductors. PCBs are generally fabricated in a xe2x80x9cfinishedxe2x80x9d form by a board fabricator and shipped lacking certain circuit elements, which are later added by an assembly house. Therefore, in a finished PCB, some circuitry must be left exposed for later attachment, usually by soldering, of further circuit elements. It is critical that such conductors retain good solderability. Copper and copper alloy conductors have previously been coated with tin or tin alloys to preserve solderability. However, electrodeposited pure tin is subject to xe2x80x9cwhiskeringxe2x80x9d, a phenomenon well known in the PCB industry. Tin alloys, such as tin/lead have been used to avoid the whiskering problem, but have the problems of expense (alloys including precious metals) or environmental unfavorability (lead).
Hot air solder leveling (xe2x80x9cHASLxe2x80x9d) has been used as a final finish for the exposed circuitry on finished PCBs. HASL produces a thick layer, e.g., 35 xcexcin to 60 xcexcin, which insures long-term solderability. The drawback to HASL is that the solder, particularly tin/lead solder, assumes a non-planar dome or dome-like shape on cooling. This non-planar shape makes subsequent placement of surface mount devices problematic.
Thin deposits (1-4 microinch (xcexcin) deposits of immersion tin have been used with silane in multilayer bonding processes to aid bonding between the metal circuitry and the laminating materials. However, this process is self-limiting as to thickness and the deposited tin rapidly alloys with copper.
Thick, pure electrodeposited tin is known to be prone to whiskering.
Rapid deposition, high build immersion tin (xe2x80x9cRDHBITxe2x80x9d has been suggested as an alternative to HASL as a final finish for PCBs. RDHBIT has been used as a replacement for HASL because it is a low cost alternative to alloys such as Ni/Au, immersion silver, immersion palladium or organic solderability preservatives.
It has been a common belief in the industry that RDHBIT is non-whiskering. However, the present inventor has discovered that this belief is erroneous, and that RDHBIT deposits from every manufacturer are subject to latent tin whiskering. When a pure or substantially pure tin deposit is subjected to a temperature of 130xc2x0 C. for a period of 70 hours in an open air oven, it develops tin whiskers. The present inventor has discovered that even RDHBIT develops whiskers. This previously unrecognized problem with immersion tin may have contributed to unexplained failures of PCBs as a result of such whiskering, and is in need of a solution.
It has now been discovered that, by applying to a substrate, e.g., to a finished PCB, a first coating of tin from an immersion tin plating solution followed by applying a cap layer of an alloy of at least two immersion platable metals onto the tin coating, a high degree of solderability can be preserved, and the problem of tin whiskering can be avoided.
In one embodiment, the present invention relates to a process for preserving solderability and inhibiting tin whisker growth of tin coated copper or copper alloy surfaces on a substrate. The process includes the steps of:
(A) preparing an immersion tin plating solution;
(B) applying the immersion tin plating solution to the substrate to form a tin coating on the surfaces;
to (C) preparing an immersion alloy plating solution containing at least two immersion-platable metals;
(D) applying the immersion alloy plating solution to the substrate to form an alloy cap layer on the tin coating. In another embodiment, the steps (B) and (D) the solution is applied by immersing the substrate in the respective solution. In another embodiment, the immersion platable metals are present in the form of metal salts.
In one embodiment, the substrate is a finished printed circuit board. In another embodiment, the immersion-platable metals are selected from tin, silver, bismuth, copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium and cobalt. In another embodiment, the at least two immersion platable metals comprise tin and silver.
In one embodiment, the tin coating has a thickness in the range from about 20 microinches to about 300 microinches. In another embodiment, the tin coating has a thickness in the range from about 40 microinches to about 60 microinches.
In one embodiment, the alloy cap layer has a thickness in the range from about 1 microinch to about 30 microinches. In another embodiment, the alloy cap layer has a thickness in the range from about 2 microinches to about 10 microinches.
In one embodiment, the step of applying the immersion tin plating solution includes a plating time in the range of about 5 to about 60 minutes.
In one embodiment, the step of applying the immersion alloy plating solution includes a plating time in the range of about 1 to about 10 minutes.
In one embodiment, the immersion tin plating solution comprises a stannous salt, an acid selected from mineral acids, carboxylic acids and hydrocarbyl-substituted sulfonic acids, a complexing agent and water. In another embodiment, the stannous salt is a stannous salt of a hydrocarbyl-substituted sulfonic acid and the acid is the hydrocarbyl-substituted sulfonic acid.
In one embodiment, the immersion alloy plating solution comprises at least two immersion-platable metal salts, an acid selected from mineral acids, carboxylic acids and hydrocarbyl-substituted sulfonic acids, a complexing agent and water. In another embodiment, each of the at least two immersion platable metal salts is a salt of a hydrocarbyl-substituted sulfonic acid and the acid is the hydrocarbyl-substituted sulfonic acid.
In one embodiment, the process for preserving solderability and inhibiting tin whisker growth of tin coated copper or copper alloy surfaces on a substrate, comprising the steps of:
(A) preparing an immersion tin plating solution;
(B) applying the immersion tin plating solution to the substrate to form a tin coating on the surfaces;
(C) preparing an immersion alloy plating solution containing tin and silver;
(D) applying the immersion alloy plating solution to the substrate to form a tin/silver alloy cap layer on the tin coating. In another embodiment, the tin/silver alloy cap layer comprises from about 50 wt % to about 98 wt % tin and from about 50 wt % to about 2 wt % silver. In another embodiment, the tin/silver alloy cap layer comprises from about 80 wt % to about 95 wt % tin and from about 20 wt % to about 5 wt % silver.
In one embodiment, the printed circuit board includes electrical circuitry formed on an outer surface of the printed circuit board, the circuitry comprising copper or a copper alloy; a final finish on the circuitry, the final finish comprising a coating of tin on the copper or copper alloy circuitry; and an alloy cap layer on the tin coating, the alloy cap layer comprising at least two immersion-platable metals. In another embodiment, the alloy cap layer comprises an alloy of tin and silver.